Methylmercury (MeHg) is an environmental agent introduced to humans primarily via the ingestion of contaminated seafood. Prenatal MeHg exposure induces neurodevelopmental deficits, at doses below those causing systemic toxicity. One potential toxicological mechanism for this and other environmental chemicals likely involves reactive oxygen species (ROS) formation, as the fetus has low antioxidative protection. This toxicity may involve oxidative DNA lesions, the most prominent of which is 8-oxoguanine (8-oxoG), which can result in mutations or altered gene transcription. The latter is potentially relevant to developmental pathologies. Repair of 8-oxoG is carried out by the base excision (BER) and transcription-coupled (TCR) repair pathways. We hypothesize that ROS-mediated oxidative DNA damage in fetal brain modifies specific gene expression levels, contributing to postnatal neurodevelopmental deficits. The progeny of mouse models with genetic alterations in 8-oxoG repair activity will be assessed for neurodevelopmental deficits after in utero exposure to MeHg. Oxoguanine glycosylase 1 (ogg1) knockout (BER-deficient) and Cockayne syndrome B (CSB) knockout (TCR-deficient) mice will be tested as repair-deficient models, while transgenic mice expressing highly active bacterial formamidopyrimidine glycosylase (Fpg) will be genetically engineered and employed as a BER-enhanced model. Fetal ROS and 8-oxoG, specific gene regulatory targets for oxidation and associated gene expression changes in utero will be analyzed in the same fetal brains, and assessed with respect to postnatal CMS pathologies in the progeny. 8-OxoG will be quantified by HPLC with electrochemical detection, oxidation of target gene regulatory elements will be characterized by ligand- mediated PCR, gene expression by microarray analysis, and neurodevelopmental deficits by behavioral tests. These studies will provide mechanistic insight into the fetal origin of environmentally-induced neurodevelopmental defects, and the role of DNA repair activity as a risk factor.